The present invention relates to a semiconductor laser and, more particularly, to a high-power laser having an oscillation wavelength in the range of 600 to 700 nm suitable for use in an optical information system or a semiconductor laser having an oscillation wavelength in the range of 1.47 to 1.62 xcexcm suitable for use in an optical communication system.
Currently, a reliable high-power laser is needed for writing information to an optical disk or a magnetooptical disk. The semiconductor laser is required to operate stably in a fundamental mode for a long time. An insulating coating film is formed on a semiconductor surface forming the radiation facet of the semiconductor laser to increase external differential quantum efficiency by obtaining a proper reflectance and to prevent the reduction of maximum output due to the increase of threshold carrier density. Many lasers use a silicon oxide thin film as an antireflection film to be formed on one of the surfaces of a resonator, and a layered film comprised of a silicon oxide thin film and a hydrogenated amorphous silicon thin film as a high-reflection film to be formed on the other surface. These coating films are mentioned in T. Uasa et al., APPLIED PHYSICS LETTERS, Vol. 34, pp. 685.
Wavelength division multiplexing of optical communication has been practically applied to an optical communication system to deal with further enhanced large-capacity transmission. Wavelength division multiplex optical communication employs a modulator-integrated light source formed by combining an optical modulator and a semiconductor laser, i.e., a light source, in one chip. The radiation facet of the modulator-integrated light source is coated with an antireflection film. These coating films are mentioned in K. Kudo et al., ELECTRONICS LETTERS, Vol. 34, No. 20, pp. 1946.
The high-reflection coating film which has been used by many conventional semiconductor lasers is a film comprised of a silicon oxide film and a hydrogenated silicon thin film and having periodic construction. The high-reflection coating film is designed so as to have a necessary reflectance. If a high-power laser having an oscillation wavelength of 1 xcexcm or below and capable of emitting light having a density of several megawatts per square centimeter is driven continuously for a constant-power operation, the light absorption of the high-reflection film of such a coating film increases sharply with time and the laser is unable to operate in a constant-power mode.
The hydrogenated amorphous silicon film is heated by heat generated by light absorption in laser operation and hydrogen atoms contained in the hydrogenated amorphous silicon film and bonded to silicon atoms are separated from silicone atoms. Consequently, optical bandgap narrows and the light absorption of the amorphous silicon film increases. Thus, the effective quantum efficiency decreases and thereby an operating current increases. Thus, such a high-reflection coating film is unsuitable for the high-power laser.
A silicon nitride thin film may be used to solve such a problem. However, a silicon nitride thin film has a refractive index of about 1.95 or below, which is nearly equal to the refractive index of a silicon oxide thin film. Therefore, when the silicon nitride thin film used as a facet coating film, the number of layers must be five periods or above, which is twice as large as the number of layers. When the hydrogenated amorphous silicon thin film is used, a predetermined reflectance can be obtained by a film of a periodic structure of two or three periods. The allowable range of thickness variation of the silicon nitride thin film is as narrow as xc2x16%.
It is a first object of the present invention to provide a semiconductor radiative device capable of stably operating for a high-power light-emitting operation for a long period of operation in an operating mode in which oscillation wavelength is in the range of 600 nm to 1 xcexcm and light having a density of several megawatts per square centimeter is emitted.
The semiconductor radiative device can be easily fabricated and operating current does not increase after the semiconductor radiative device is used for a long time. Thus, the semiconductor radiative device has a long service life and high reliability.
A modulator-integrated light source for wavelength division multiplex optical communication needs an antireflection film having a reflectance of 0.1% or below for coating the radiative facet. When the degree of wavelength division multiplexing is large and light of wavelengths in a wide wavelength range is used, it is difficult for a single antireflection film to meet a requirement that the reflectance of the antireflection film with the light of those wavelengths be 0.1% or below.
Another object of the present invention is to provide an antireflection film capable of reflecting light of wavelengths in an oscillation wavelength range of 1.47 xcexcm to 1.62 xcexcm used for wavelength division multiplex communication.
According to a first aspect of the present invention, a semiconductor radiative device includes a layered film comprised of a low-refraction first dielectric film and a high-refraction second dielectric film having a refraction index greater than that of the first dielectric film, and formed on at least one of the facets of an optical cavity, wherein the high-refraction second dielectric film is an amorphous dielectric film containing silicon, hydrogen and nitrogen.
According to a second aspect of the present invention, a semiconductor radiative device includes a layered film comprised of a low-refraction first dielectric film and a high-refraction second dielectric film having a refraction index greater than that of the first dielectric film, and formed on at least one of the facets of an optical cavity, wherein the high-refraction second dielectric film is an amorphous dielectric film containing silicon, hydrogen and nitrogen, and having a refractive index of 2.5 or above with light of a wavelength to be emitted by the semiconductor radiative device.
According to a third aspect of the present invention, a semiconductor radiative device includes a layered film comprised of a first dielectric film and a high-refraction second dielectric film having a refraction index greater than that of the first dielectric film, and formed on at least one of the facets of an optical cavity, wherein the high-refraction second dielectric film is an amorphous dielectric film containing silicon, hydrogen and nitrogen, and having a refractive index of 2.5 or above and an extinction coefficient of 0.005 or below with light of a wavelength to be emitted by the semiconductor radiative device.
The amorphous dielectric film containing silicon, hydrogen and nitrogen is used as a component member of the high-reflection film (HR film) or the antireflection film (AR film). The HR film or the AR film is designed satisfactorily by an ordinary design method.
The HR film is formed by layering a high-refraction film and a low-refraction film. The amorphous dielectric film according to the present invention having a high refractive index is very preferable. The layering period is determined taking into consideration a desired reflectance and degree of light absorption. Practically, the layering period is in the range of two periods to five periods. A representative number of periods is three.
The layered coating film including an amorphous dielectric film according to the present invention is suitable for use in a compound semiconductor radiative device having an oscillation wavelength of 600 nm or above. The layered coating film is suitable for use as a HR film for a semiconductor radiative device having an oscillation wavelength in a short-wavelength region of 600 to 680 nm. The amorphous dielectric film maintains a high reflectance and has a small light absorption coefficient with light of the oscillation wavelength of the semiconductor radiative device. Thus, the amorphous dielectric film enables the constant-power operation of the semiconductor radiative device.
The thickness of an AR film as the dielectric film of a semiconductor radiative device having an oscillation wavelength in the long-wavelength region of 1.3 to 1.55 xcexcm can be easily controlled; that is an allowable range for the thickness of the dielectric film is wide.
It is particularly preferable that the amorphous dielectric film does not contain any oxygen. In some cases, the amorphous dielectric film contains oxygen in an oxygen content of, for example, about 0.1% in the process of forming the amorphous dielectric film. Such an oxygen content is insignificant. However, it is desirable to avoid oxygen mixing in or remaining in the amorphous dielectric film because oxygen contained in the amorphous dielectric film reduces the refraction index of the amorphous dielectric film.